Solder self-alignment is a technique for aligning devices, such as semiconductor chips, to carriers, such as packages. Solder is predeposited on the carriers typically to lithographic precision to form bond pads. The devices are placed on bond pads to form a precursor structure, which is then placed in a solder reflow oven where the solder pads are heated to a liquidous state. The resulting surface tension in the liquid solder pulls the devices into alignment with the bond pads and thus the carriers.
Solder self-alignment has also been used in optoelectronic device manufacture. In these applications, optical components, such as an active device, e.g., laser chip, or passive devices, e.g., lenses or filters, are placed on bond pads, which have been predeposited on optical benches or submounts. Then, these precursor structures are placed in a solder reflow oven where the solder is heated to a liquidous state. The optical components are then pulled into alignment by surface tension on the optical benches.
With well-characterized processes, optical component alignment using solder self-alignment techniques is effective to align the optical components relative to the benches to accuracies of about 10 micrometers (xcexcm). As a result, it competes with other passive alignment processes, such as registration features and/or installation by pick-and-place machines, in high volume manufacturing processes. The advantage is that it can be implemented quickly using manual placement of the optical components on the solder pads.
The problem with solder self-alignment, however, is its alignment accuracy limitation. Many carrier-class fiber optic components require alignment accuracy of a few micrometers to sub-micrometer accuracy. Existing solder alignment processes generally cannot achieve such tolerances.
The present invention is directed to a process for assembling micro-optical systems, such as optoelectronic and/or fiber optic components. It uses solder self-alignment to achieve a coarse, passive alignment of optical components relative to the optical bench. The fine, final alignment, however, is performed using plastic deformation of the optical components to thereby improve the alignment of the optical components. As a result, alignment accuracies of a few micrometers to sub-micrometer are attainable, if required.
In general, according to one aspect, the invention features a process for assembling micro-optical systems. This process comprises depositing pads at locations on optical benches determined by intended engagement points between optical components and the optical benches. The optical components are then placed on these solder pads. A solder reflow process is then performed to join the optical components to the optical benches using the solder pads. During this process, self-alignment of the optical components is allowed using the solder surface tension. Finally, according to the invention, after solidification of the solder, the alignment of the optical components is improved by plastically deforming the components on the benches.
In typical applications, the solder reflow process results in the positioning of the optical components to accuracies of between 2 and 10 micrometers. Then, the final step of improving the alignment using plastic deformation results in the alignment of the components to about 1 micrometer or better. In one implementation, the plastic deformation is performed in an active alignment process. Specifically, optical signals are directed to the optical component and the optical components deformed in response to the optical signals after interaction with the optical components.
In another implementation, the step of plastically deforming the optical components comprises deforming the optical components in response to metrology data describing in the positions of the optical components relative to the benches and also so in response to the desired positions of the optical components.
To facilitate the solder attachment process, in some embodiments, solder or metal pads are predeposited on feet of the optical components. Alternatively, or in addition, solder preforms can be further placed between the optical components and the solder pads of the benches.
In one example, the optical components are placed on the solder pads manually. Vacuum wands are preferably used to manipulate the small optical components.
In alternative processes, pick-and-place machines are used, such as flip chip bonders, to place the optical components on the benches.
Further, lateral registration features are helpful in some implementations to facilitate the initial placement of the optical components on the pads. Registration features can also be further used to control the subsequent self-alignment process. These registration features are formed on the bench surface in one implementation. Alternatively, separate templates are used.
In some implementations, magnetic fixturing is used at least after the placement of the optical components on the optical bench to hold the optical components on the optical bench. This is typically accomplished by providing optical components that include mounting structures that are made from a ferromagnetic material, such as nickel or iron. A magnetic field is oriented at least partially orthogonal to the bench in a downward direction toward the bench.
In general, according to another aspect, the invention also features an unpopulated bench precursor structure. This structure comprises a bench. Typically, the benches are manufactured from a temperature stable substance such as silicon or beryllium nitride, or aluminum nitride. The pads are predeposited on the bench at locations determined by desired engagement points between the optical components and the optical bench. Registration features are further provided on the bench in or near the solder pads for supporting the optical components at a predetermined position vertically on the bench.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.